Environmental impact of reservoirs: Difference between revisions
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== Upstream impacts == |
== Upstream impacts == |
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[[Image:Lake Nasser.jpg|thumb|[[Lake Nasser]] behind the [[Aswan dam]], Egypt, 5250 km², displaced 60,000 people <ref name="Thayer">''A comparative survey of dam-induced resettlement in 50 cases'' by Thayer Scudder and John Gray [http://www.hss.caltech.edu/~tzs/50%20Dam%20Survey.pdf]</ref>]] |
[[Image:Lake Nasser.jpg|thumb|[[Lake Nasser]] behind the [[Aswan dam]], Egypt, 5250 km², displaced 60,000 people <ref name="Thayer">''A comparative survey of dam-induced resettlement in 50 cases'' by Thayer Scudder and John Gray [http://www.hss.caltech.edu/~tzs/50%20Dam%20Survey.pdf]</ref>]] |
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=== Creation of a reservoir === |
=== Creation of a reservoir === |
Revision as of 14:53, 2 March 2012
The environmental impact of reservoirs is coming under ever increasing scrutiny as the world demand for water and energy increases and the number and size of reservoirs increases.
Dams and the reservoirs can be used to supply drinking water, generate hydroelectric power, increasing the water supply for irrigation, provide recreational opportunities and to improve certain aspects of the environment. However, adverse environmental and sociological impacts have also been identified during and after many reservoir constructions. Whether reservoir projects are ultimately beneficial or detrimental—to both the environment and surrounding human populations— has been debated since the 1960s and probably long before that. In 1960 the construction of Llyn Celyn and the flooding of Capel Celyn provoked political uproar which continues to this day. More recently, the construction of Three Gorges Dam and other similar projects throughout Asia, Africa and Latin America have generated considerable environmental and political debate.
![](http://upload.wikimedia.org/wikipedia/commons/thumb/e/eb/Wachusett-dam.jpg/220px-Wachusett-dam.jpg)
Upstream impacts
![](http://upload.wikimedia.org/wikipedia/commons/thumb/4/4a/Lake_Nasser.jpg/220px-Lake_Nasser.jpg)
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Creation of a reservoir
The damming of a river creates a reservoir upstream from the dam. The reservoir waters spill out into the surrounding environments, flooding the natural habitats that existed before the dam’s construction. To date, over 400,000 km2 of the earth have been flooded due to damming. The newly created reservoir has more surface area than the river would have had, and therefore more evaporation occurs than previously. This can lead to a loss of up to 2.1 meters in depth per year in some climates. Reservoirs can also contribute to greenhouse gas emissions.[2]
The initial filling of a reservoir floods the existing plant material, leading to the death and decomposition of the carbon-rich plants and trees. The rotting organic matter releases large amounts of carbon into the atmosphere. The decaying plant matter itself settles to the non-oxygenated bottom of the reservoir, and the decomposition—unmitigated by a flow pattern that would oxygenate the water—produces and eventually releases dissolved methane.
Fragmentation of river ecosystems
A dam also acts as a barrier between the upstream and downstream movement of migratory river animals, such as salmon and trout.[3] Dams block their migration upstream to spawning areas, threatening to decrease reproduction numbers and reduce the species population. In light of this consequence, efforts have been made to allow the fish a passage upstream, and newer dams often include artificial "fishways" or "fish ladders".
Some communities have also begun the practice of transporting migratory fish upstream to spawn via a barge.[3] Fish sometimes have difficulty migrating downstream through a dam, meaning that downstream populations are often reduced, unless the fish are able to swim safely through the dams’ spillways. Permanent inundation caused by reservoir flooding also alters the wetlands, forests and other habitats surrounding the river. Further ecosystem disruption occurs along the banks of the river and downstream. The areas surrounding riverbanks are of a particularly rich bio-diversity, supported by the natural flooding of a dam-free river. Dammed rivers reduce flood rates, and this has negative consequences on the floodplains downstream that depend on seasonal waters for survival. The comparatively invariable ecosystem created by a reservoir-river supports a far-reduced range of wildlife. Dams hold back sediments that would replenish down stream ecosystems naturally. Endemic species may or may not survive the environmental change, and new species are likely to adopt the altered habitat as a home. However, since dams change a key ecosystem to which all surrounding ecosystems have adapted, dam construction nearly always reduces wildlife diversity, for better or for worse. It also results in the loss of habitat of many living organisms.
Reservoir sedimentation
Rivers carry four different types of sediment down their riverbeds, allowing for the formation of riverbanks, river deltas, alluvial fans, braided rivers, oxbow lakes, levees and coastal shores. The construction of a dam blocks the flow of sediment downstream, leading to downstream erosion of these Sedimentary depositional environment, depositional environments, and increased sediment build-up in the reservoir. While the rate of sedimentation varies for each dam and each river, eventually all reservoirs develop a reduced water-storage capacity due to the exchange of storage space for sediment.[4] Diminished storage capacity results in decreased ability to produce hydroelectric power, reduced availability of water for irrigation, and if left unaddressed, may ultimately result in the expiration of the dam and river.[5]
Impact below dam
Riverline and coastal erosion
As all dams result in reduced sediment load downstream, a dammed river is said to be “hungry” for sediment. Because the rate of deposition of sediment is greatly reduced since there is less to deposit but the rate of erosion remains nearly constant, the water flow eats away at the river shores and riverbed, threatening shoreline ecosystems, deepening the riverbed, and narrowing the river over time. This leads to a compromised water table, reduced water levels, homogenization of the river flow and thus reduced ecosystem variability, reduced support for wildlife, and reduced amount of sediment reaching coastal plains and deltas.[5] This prompts coastal erosion, as beaches are unable to replenish what waves erode without the sediment deposition of supporting river systems. Channel erosion of rivers has its own set of consequences. The eroded channel could create a lower water table level in the affected area, impacting bottomland crops such as alfalfa or corn, and resulting in a smaller supply.[6]
Water temperature
The water of a reservoir is usually warmer in the winter and cooler in the summer than it would be without a dam. As this water flows into its river, the altered temperature also affects the temperature of the river. This impacts the plant and animal life present in both the reservoir and the river, often creating environments that are unnatural to the endemic species. A substantial decrease in fishing success for Atlantic salmon and Sewin on the River Towy has been shown to be related to the reduced water temperature caused by the bottom water discharge from Llyn Brianne, a dam created in the 1960s. Recent shifts in the populations of fish in the Snake River and Klamath region have, for example, prompted the renewal of research and restoration projects aimed at relieving the stress of temperature change.
Effects beyond the reservoir
Effects on humans
Diseases
Whilst reservoirs are helpful to humans, they can also be harmful as well. One negative effect is that the reservoirs can become breeding grounds for disease vectors. This holds true especially in tropical areas where mosquitoes (which are vectors for malaria) and snails (which are vectors for Schistosomiasis) can take advantage of this slow flowing water.[7]
![](http://upload.wikimedia.org/wikipedia/commons/thumb/9/9b/10.34748W_13.10430N.png/220px-10.34748W_13.10430N.png)
Resettlement
Dams and the creation of reservoirs also require relocation of potentially large human populations if they are constructed close to residential areas. The record for the largest population relocated belongs to the Three Gorges dam built in China. Its reservoir submerged a large area of land, forcing over a million people to relocate. "Dam related relocation affects society in three ways: an economic disaster, human trauma, and social catastrophe", states Dr. Michael Cernea of the World Bank and Dr. Thayer Scudder, a professor at the California Institute of Technology.[1]
Effects on flood-dependent ecology/agriculture
In many developing countries the savanna and forest ecology of the floodplains depend on seasonal flooding from rivers. Also, flood recession cropping is practiced extensively whereby the land is cultivated taking advantage of the residual soil moisture after floods recede. Dams attenuate floods which may affect the ecology and agriculture seriously.
![](http://upload.wikimedia.org/wikipedia/commons/thumb/e/ed/BaluchNomads.jpg/220px-BaluchNomads.jpg)
Case studies
- The Manatali reservoir formed by the Manantali dam in Mali intersects the migration routes of nomadic pastoralists and destroyed 43000 ha of savannah, probably leading to overgrazing and soil erosion elsewhere. Further, the reservoir destroyed 120 km² of forest. The depletion of groundwater aquifers, which is caused by the suppression of the seasonal flood cycle, is damaging the forests downstream of the dam.[9]
- In the case of the Lower Omo River, Ethiopia, hundreds of thousands of nomads will be affected because the flood recession agriculture on which their livelihoods are based would disappear once the Gibe III dam will be completed.
- After the closure of the Kainji dam, Nigeria, 50 to 70 per cent of the downstream area of flood-recession cropping was lost.[10]
Effects on the Earth itself
Reservoirs may contribute to changes in the Earth's climate. Warm climate reservoirs generate methane, a greenhouse gas when the reservoirs are stratified, in which the bottom layers are anoxic (i.e. they lack oxygen), leading to degradation of biomass through anaerobic processes.[11] In some cases, where flooded basins are wide and biomass volumes are high the amount of biomass converted to methane results in pollution potential 3.5 times more than an oil-fired power plant would for the same generation capacity.[12]
See also
References
- ^ a b A comparative survey of dam-induced resettlement in 50 cases by Thayer Scudder and John Gray [1]
- ^ Hydroelectric power's dirty secret revealed - earth - 24 February 2005 - New Scientist
- ^ a b Mann, Charles C (2000). "Can Science Rescue Salmon?". Science, New Series. 289 (5480): 716–719.
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ignored (help) - ^ Silenced Rivers: The Ecology and Politics of Large Dams, by Patrick McCully, Zed Books, London, 1996. ISBN 1856499022
- ^ a b Reservoir Sedimentation Handbook; Morris, Gregory & Fan, Jiahua; McGraw-Hill Publishers; 1998.
- ^ Sedimentation Engineering; American Society of Civil Engineers Committee; American Society of Civil Engineers Headquarters; 1975.
- ^ William R. Jobin, 1999. Dams and Disease: Ecological Design and Health Impacts of Large Dams, Canals, and Irrigation Systems, Taylor & Francis, ISBN 0419223606 [2]
- ^ ILRI, 1982. Modern interferences in traditional water resources in Baluchistan. In: Annual Report 1982, pp. 23-34. ILRI, Wageningen, The Netherlands. Reprinted in Water International 9 (1984), pp. 106- 111. Elsevier Sequoia, Amsterdam. Also reprinted in Water Research Journal (1983) 139, pp. 53-60. Download from : [3] , under nr. 10, or directly as PDF : [4]
- ^ A. deGeorges and B.K. Reilly, 2006. Dams and large scale irrigation on the Senegal river: impacts on man and the environment. UNDP Human Development Report. On line: http://hdr.undp.org/en/reports/global/hdr2006/papers/DeGeorges%20Andre.pdf
- ^ C.A.Drijver and M.Marchand, 1985. Taming the floods. Environmental aspects of the floodplain developments of Africa. Centre of Environmental Studies, University of Leiden, The Netherlands.
- ^ Climate Change and Dams: An Analysis of the Linkages Between the UNFCCC Legal Regime and Dams.
- ^ http://www.newscientist.com/article/dn7046 Hydroelectric Power's Dirty Secret
External links
- Rivers No More: The Environmental Effects of Large Dams at International Rivers (an excerpt for Rivers No More: The Environmental Effects of Large Dams)
- World Commission on Dams